This invention relates to a coaxial engine starter and, more particularly, to a coaxial engine starter for a vehicular engine.
According to the conventional coaxial engine starter 10 shown in FIG. 1, a d.c. electric motor 1 has a hollow armature rotary shaft 2 and a plunger rod 4 of a solenoid switch 3 positioned at the rear end of the d.c. motor 1 is inserted into an inner passage 2a of the armature rotary shaft 2 so that the inserted front end of the plunger rod 4 abuts against the rear end of an output rotary shaft 5 coaxially disposed at the front end of the armature rotary shaft 2 and inserted into the inner passage 2a of the armature rotary shaft 2, whereby the output rotary shaft 5 can be pushed forward.
It is seen that a sun gear 10a is formed on the outer circumference of the front end of the armature rotary shaft 2 and a plurality of planetary gears 10b are in mesh with the sun gear 10a. These planetary gears 10b also mesh with an inner gear 9 formed in the inner circumferential surface of the front frame 13 and are rotatably supported by shafts 11 secured on a carrier 8. The sungear 10a, the planetary gears 10b, the inner gear 9, the shafts 11 and the carrier 8 together constitute a planetary speed reduction gear 10 which reduces the rotational speed of the armature rotary shaft 2. On the output rotary shaft 5, an over-running clutch 8 is fit, of which clutch inner member 8b is engaged with the output rotary shaft 5 by helical splines 5c of spline formation portion 5b having an outer diameter larger than the inner diameter of the inner passage 5a of the armature rotary shaft 2, so that the output rotary shaft 5 is allowed to axially slide while being rotated by the clutch inner member 8b. On the front end of the output rotary shaft 5, a pinion 6 which engages and disengages relative to the engine ring gear (not shown) is mounted. When the output rotary shaft 5 is moved forward, the pinion 6 meshes the engine ring gear to rotate the engine.
However, in the coaxial engine starter of the above construction, the rear end surface of the large-diameter. spline formation portion 5b of the output rotary shaft 5 directly bumps and exerts a thrust against the front end surface of the armature rotary shaft 2 when the output rotary shaft 5 which has moved forward by the plunger rod 4 of the solenoid switch 3 is returned to its original position by the action of the spring 12. Also, the rear end surface of the clutch inner member 8b and the armature rotary shaft 2 rotate relative to each other with a significant slip therebetween. More particularly, since the rotation of the output rotary shaft 5 of this coaxial engine starter is transmitted to an engine ring gear (not shown) through the pinion 6, the output rotary shaft 5 is reversely driven to rotate at a high speed by the engine ring gear until the pinion 6 disengages the ring gear after the engine has been started. Therefore, the high speed rotation of the output rotary shaft 5 driven by the engine is interrupted by the one-way clutch mechanism 8 in order not to be transmitted to the armature rotary shaft 2 to protect the d.c. motor. However, since the rotational speed of the clutch inner member 8b engaged with the output rotary shaft 5 through the helical spline and the rotational speed of the armature rotary shaft 2 are greatly different from each other, a very rapid wear is observed at the interface between the clutch inner member 8b and the output rotary shaft 5. Moreover, the output rotary shaft 5 hits against the armature rotary shaft 2 with a significant impact and slips therebetween; this interface between the output rotary shaft 5 and the armature rotary shaft 2 is also very easily damaged. For the above reasons, the conventional coaxial engine starter has only a relatively short operating life which is not entirely satisfactory.
Further, the front bracket 13 for mounting the starter to an engine must be configured and dimensioned in accordance with the configuration and the dimension of the mounting portion of the different type of the engine. Therefore, several types of the front bracket must be designed separately for each model of the engine. When an already assembled starter is to be used with a different engine, the front bracket 13 must be disassembled and the several parts within the casing such as the drive force transmitting mechanism 9 and the output rotary shaft 5 must be disassembled and the starter must be reassembled with another front bracket which fits for that particular engine. During the disassembling and reassembling of the starter in order to change the front bracket, the over-running clutch mechanism 8 and the speed reduction planetary gear 10 are exposed and must be disassembled, so that these parts are quite easily polluted by dust or foreign matter and that those parts connected to the d.c. motor must be re-assembled.
Thus, the conventional coaxial engine starter has problems in operating life, easy assembly, keeping the parts clean, etc.